This invention relates to a process for forming electrically conductive metallic patterns on a thin polyolefin film substrate. More particularly, this invention relates to the formation of resonant RF-tuned printed circuits formed on a thin polyolefin film which are particularly useful in electronic security and article surveillance detection systems.
Presently, electronic security systems are utilized to detect unauthorized removal of articles from a protected area. These systems utilize radio waves, microwaves or a magnetic field generated within a confined area through which all articles from a shopping area must pass. A special tag is attached to the article which is sensed by a receiving system to signify the unauthorized removal of the article. When the sensing system does not sense the presence of the special tag within the confined area, then the removal of the article is authorized by virtue of its being paid for and the tag has been either removed from the article at the checkout counter or has been deactivated at the checkout counter.
A preferred special electronic tag utilizes a technology based on tuned circuits which operate in the radio frequency range. To render the tuned circuit functional at the desired frequency, a discrete inductor (L) and a discrete capacitor (C) are connected together. The reusable resonant tag uses discrete capacitor and inductor components which are connected to form the tuned inductor-capacitor (LC) circuit. Prior to the present invention, the capacitor and inductor have been formed by conventional fabrication methods for forming printed circuits including selective use of laminated substrates having an interior dielectric layer laminated on both surfaces with a conductive composition such as aluminum or copper. The conductive layers are printed with an etchant resistant material in the form of the desired circuit and, after etching, the remaining conductive material is now in the form of the desired circuit. Such a conventional process is disclosed, for example, in U.S. Pat. Nos. 3,913,219 and 4,369,557. Further examples of resonant circuit tags are disclosed in U.S. Pat. Nos. 3,967,161; 4,021,705; 3,810,147 and 3,863,244. Any process for producing disposable resonant tag circuits must be capable of producing satisfactorily functioning resonant tag circuits at high volume and low cost with accurate tolerances so that uniform electrical properties are obtained from tag to tag. In resonant tag circuits, it is desirable to produce tags that operate at specific frequencies. Specific frequencies can be obtained by varying L and/or C based on the equation: ##EQU1## In general, it is also desirable to have a sharp resonance curve where there is a large change in impedance over a narrow frequency range in order to provide the desired selectivity to discriminate between tuned circuits and environmental interferences.
The sharpness of the resonance curve is usually described by a quality factor called "Q" which can be defined as the ratio of the reactance of either the coil or the capacitor at resonant frequency to the total resistance. It is also a measure of the reactive power stored in the tuned circuit to the actual power dissipated in the resistance of the circuit. The higher the "Q", the greater the amount of energy stored in the circuit compared with the energy lost in the resistance during each cycle. A circuit with a higher "Q" has greater sensitivity in responding to the detector field due to less energy loss within the circuit. Therefore, it is generally desirable to have a resonant tag circuit with a high "Q" factor.
Mathematically: ##EQU2## Where X.sub.L =Inductive reactance
X.sub.C =Capacitive reactance PA0 L=Inductance PA0 C=Capacitance PA0 f=Frequency PA0 R=Resistance PA0 (b) Increasing the inductance (L) PA0 (c) Reducing the capacitance (C)
Combining equations 2 and 3: ##EQU3## which indicates "Q" can be improved by: (a) Lowering the resistance (R)
The "Q" factor is also related to the nature of the dielectric film in the resonant tag circuit which means, the dielectric loss of the substrate should be minimized to improve the "Q" factor. This dielectric loss is normally referred to as the dielectric dissipation factor of the capacitor. Many polymeric films such as polyimide film or polyester film cannot be utilized to form resonant tag circuits since their dielectric properties are inappropriate at the desired frequencies.
Federal Communication Commission requirements dictate that the frequency and power level of the swept electromagnetic waves be held within fairly close tolerances, which in turn, requires that the resonant tag circuits have a relatively high "Q" factor and the resonant frequencies fall within a narrow range in order to assure that they will be reliably detected by the system.
With the prior art techniques such as those disclosed in U.S. Pat. Nos. 3,913,219 and 4,369,557 which use printed circuit etching techniques combined with printing of an etch-resistant pattern on opposite surfaces of a metal foil laminated onto an insulative material, it is difficult to mass-produce resonant circuits in relatively high quantities within the desired resonant frequency tolerances of 10% (.+-.5% of the center of resonant frequency desired), due to variations of the printing and etching rates.
In processes utilizing a step wherein a metal coating of relatively uniform thickness is etched to remove it from a substrate, control of the process is difficult, particularly when the desired circuit includes thin conductive lines. With such circuits, it is essential that etching compositions, metal thicknesses, etching times and other process parameters be uniformly and precisely controlled in order to maintain the integrity of the desired circuit. This problem of maintaining precise control of the process parameters is not nearly so critical in additive processes, since in additive processes one must attain a certain minimum metal deposition (thickness) and further metal deposition beyond the minimum does not adversely affect the quality of the circuit thus produced. The requirement for precise process control in the subtractive etching type processes serves to limit the volume production of circuits since process down-time frequently occurs when the process parameters are found to be outside of the critical process parameters necessary for producing satisfactory products. The use of a subtractive etching type process is also undesirable since the metal removed from the laminated substrate is lost and the cost of the process is thereby increased. This is particularly true when it is desirable to use highly conductive and expensive metals such as copper, silver or gold. As a practical matter, the subtractive etching process is only commercially feasible when using less conductive, inexpensive metals such as aluminum to form the laminated substrate from which the circuits are produced.
It is also known in the prior art to produce metal coatings on plastic parts such as molded plastics used for automotive parts (e.g., knobs, trim, etc.) which involves the steps of treating a plastic substrate so that it is capable of accepting a catalyst compound that promotes electroless metal deposition. The thus-activated plastic substrate then is immersed in an electroless metal bath to form a conductive, metal coating on the substrate. Additional metal coatings can be formed on the electroless metal surface by electroplating if desired. In these processes, the plastic substrates have a large mass and are relatively thick and therefore are capable of withstanding severe process conditions such as exposure to strong solvents and etching acids at elevated temperatures to condition the surface so that catalyst compounds can be deposited on the plastic surface relatively quickly.
It is also known to form printed circuits by using a variety of masking and plating techniques as disclosed for example in U.S. Pat. Nos. 4,293,592; 4,322,457 and 4,354,911.
Prior to the present invention, it has been known to form printed circuits on relatively thin and flexible polyester or polyimide substrates as disclosed for example in U.S. Pat. No. 4,261,800. The resultant products are useful as flexible circuits where highly efficient utilization of space is essential. However, the technology used to form printed circuits on these substrates is not generally useful for forming printed circuits on polyolefin substrates since the surface chemistry of polyolefins is far less reactive as compared with polyesters or polyimides. Furthermore present technology for producing printed circuits is not applicable to thin polyolefin films since the film has far less mechanical strength and resistivity to chemical baths and solvents. Thus, at the present time, there has not been available a totally satisfactory process for forming electrically conductive circuits on thin film polyolefin substrates by additive plating technology. Thin film plastic substrates are difficult to utilize in an additive process for producing a printed circuit thereon. This is due primarily to the requirement of catalyzing the surface of the thin plastic film so that subsequent electroless deposition of metal can be achieved which is adherent and has sufficient adhesion that it will not flake off under flexing conditions. The conventional means for implanting a catalyst composition on the surface of the plastic substrate utilizes severe process conditions. In the case of thin film plastic substrates, the mechanical integrity of the film would be destroyed utilizing conventional process parameters. For example, hot solvents such as chlorinated hydrocarbons, dimethylformamide or cyclic ethers would destroy the mechanical integrity of most thin film plastic substrates.
These problems with thin films are compounded when it is desired to process the thin film continuously in a roll-to-roll process; that is, in a process wherein an untreated thin film is unwound from a feeder roll, passed through the treatment baths and wound up on a take-up roll. During the individual processing steps, the thin film may expand or contract depending upon the nature of the liquid medium, which maybe at variable temperatures, through which it is passed. In any event, it is extremely difficult to effect the desired surface treatment of the thin film strip in order to affix thereon a circuit of the desired design, particularly when a portion of the circuit is affixed to opposing surfaces of the thin film and when the circuit portions on the opposing surfaces must be in proper alignment so that the desired overall circuit functions can be attained. The surface treatment must be sufficiently effective to modify the surface to effect printing but must be relatively mild so that the mechanical integrity of the film is maintained and the desired circuit pattern can be formed thereon. These process requirements are necessary for each film treatment stage.
It has been proposed in U.S. Pat. No. 4,006,047 to utilize a class of organic palladium moiety complexes to deposit palladium catalyst on a thin film substrate in order to catalyze subsequent electroless metal deposition and to avoid excessively high temperatures which damage the thin film while activating the catalyst. This patent discloses a technique for plating on polyolefin film which utilizes highly flammable solvents and moisture sensitive organo-palladium complexes. Furthermore, the palladium complexes are easily oxidized and thus rendered useless as a catalyst for electroless metal deposition unless plated immediately. Therefore, this process does not allow for printing a circuit image mask on the film prior to electroless metal deposition.
Accordingly, it would be desirable to provide a means for utilizing an additive process for forming printed circuits on a thin film polyolefin substrate under conditions which retain the catalytic activity and mechanical integrity of the polyolefin film. Furthermore, it would be desirable to provide such a process, which is capable of producing printed circuits within the close tolerances normally required at high volumes in a roll-to-roll process, in order to provide commercial incentive for utilizing the process. Furthermore, it would be desirable to provide such a process which eliminates the need for etching relatively thick layers of metal from the metal-plastic laminates now employed to produce circuits.